The Directory Schema.

In addition to the object classes and attribute types defined
in the standard, an organisation may wish to define its own objects and attributes.
It also needs to structure the DIT to meet its specific requirements.

The directory Schema provides the means to do so. !! Link to this from defintiion page on
attributes and objects!!
The directory schema is the set of rules and constraints governing the DIT structure, and
the object classes, attribute types and syntaxes in the DIB. It therefore enables the
integrity of the DIB and DIT to be maintained when objects are deleted, inserted or
modified.

Obviously, if non-X.500 objects and attributes are defined, they can be accessed within
the organisation or within closed user groups, but not universally.
Like DIB, the Directory Schema can itself be distributed.